Gas sensors are employed in a variety of applications requiring qualitative and quantitative gaseous determinations. In the automotive industry, it is well known that the oxygen concentration in the automobile exhaust has a direct relationship to the engine air-to-fuel ratio. Oxygen gas sensors are employed within the automobile internal combustion control system to provide accurate exhaust gas oxygen concentration measurements for determination of optimum combustion conditions, maximization of efficient fuel usage, and management of exhaust emissions.
Generally, the electrochemical type of oxygen sensor employed in automotive applications utilizes a thimble shaped electrochemical galvanic cell to determine, or sense the relative amounts of oxygen present in the exhaust stream, an example being U.S. Pat. No. 3,844,920 to Burgett et al. This type of oxygen sensor is generally known and used throughout the automotive industry, and comprises an ionically conductive solid electrolyte material which is typically yttria stabilized zirconia, a porous electrode coating on the exterior of the solid electrolyte exposed to the exhaust or measuring gas, and a porous electrode coating on the interior of the solid electrolyte exposed to a known concentration of reference gas.
The gas concentration gradient across the solid electrolyte produces a galvanic potential which is related to the differential of the partial pressures of the gas at the two electrodes by the Nernst equation: E=AT ln[P.sub.1 /P.sub.2 ], where E is the galvanic voltage, T is the absolute temperature of the gas, P.sub.1 /P.sub.2 is the ratio of the partial pressures of the reference gas at the two electrodes, and A=R/4F, where R is the universal gas constant and F is the Faraday constant Thus, the oxygen sensing device determines the oxygen concentration in the exhaust gas, by measuring this galvanic potential generated between the reference and measuring electrodes.
Two extremely important functional requirements of an automotive exhaust oxygen sensor are (1) the ability of the sensor assembly to maintain electrical contact between the measuring and reference electrodes and the external measuring equipment, while (2) preventing leakage of the exhaust gases into the air reference chamber (and correspondingly to the reference electrode) of the solid electrolyte body. It is also required that these functions be strictly maintained over a wide range of temperatures with materials having diverse thermal expansion characteristics.
Various means have been employed in the past to achieve good sealing and electrical contact within the sensor assembly over a wide range of temperatures. Most of these means utilize some type of spring member to achieve the requisite gas-tight seal and electrical contact, and can generally be divided into two broad categories, In the first category, the spring member is placed inside the gas sensor shell near the region where the hot exhaust gases flow, typically between an insulator and the positive electrical contact on the solid electrolyte body. This design requires a spring material which can tolerate the high temperatures experienced as the exhaust gases flow over the solid electrolyte body. Such materials are relatively expensive and do not provide optimum spring properties In addition, typically, such an arrangement subjects the spring member to extremely high loading forces during assembly of the components, resulting in a significant loss in its spring force.
Alternatively, the spring member has been provided away from the extremely hot section of the sensor assembly where the exhaust gases flow. However, although this design permits the use of less costly materials for the spring member, it is still required that the full assembly force be applied to the spring member, thereby again diminishing the spring force of the member.
Therefore, it would be desirable to provide a means for assembling these types of oxygen sensors which does not employ a spring member to ensure effective sealing and electrical contact within the sensor.
A significant, additional drawback also exists with regard to the use of either of these typical arrangements for achieving adequate sealing and electrical contact In order to functionally test the assembly, the typical design requires the assembly of the entire sensor, including the wiring and incorporation of all metal components. This is problematic since the wiring adds complexity to the high volume automatic testing of these assemblies. In addition, it is extremely difficult to salvage some of the components if a defect is detected at such an advanced stage in the assembly of the exhaust oxygen sensor.
Therefore, it would also be desirable to provide a means for assembling these automotive exhaust oxygen sensors which would permit the testing of a functional subassembly prior to final build-up of the sensor assembly, thereby maximizing the amount of salvageable material upon detection of a defect. Further, for automotive applications particularly, it is required that the oxygen sensor be rugged, reliable, and readily manufacturable at a low cost, therefore accordingly the assembly means should be amenable to these production requirements.